Flow batteries having a pressure-balanced electrochemical cell stack and associated methods

ABSTRACT

Electrolyte solution circulation rates in a flow battery can impact operating performance. Although adjusting the circulation rates can allow improved performance to be realized, it can be difficult to levelize circulation rates over multiple electrochemical cells of an electrochemical cell stack due to a non-uniform pressure drop that occurs at an outlet of each electrochemical cell. Accordingly, flow batteries capable of realizing improved operating performance can include: an electrochemical cell stack containing a plurality of electrochemical cells in electrical communication with one another; an inlet manifold containing an inflow channel fluidically connected to an inflow side of each of the electrochemical cells; an outlet manifold containing an outflow channel fluidically connected to an outflow side of each of the electrochemical cells; and an insert disposed in the outflow channel. The insert has a variable width along a length of the outflow channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD

The present disclosure generally relates to energy storage and, morespecifically, to methods and systems configured for regulatingcirculation of an electrolyte solution through an electrochemical cellstack of a flow battery.

BACKGROUND

Electrochemical energy storage systems, such as batteries,supercapacitors and the like, have been widely proposed for large-scaleenergy storage applications. Various battery designs, including flowbatteries, have been considered for this purpose. Compared to othertypes of electrochemical energy storage systems, flow batteries can beadvantageous, particularly for large-scale applications, due to theirability to decouple the parameters of power density and energy densityfrom one another.

Flow batteries generally include negative and positive active materialsin corresponding electrolyte solutions, which are flowed separatelyacross opposing faces of a membrane or separator in an electrochemicalcell containing negative and positive electrodes. The terms “membrane”and “separator” are used synonymously herein. The flow battery ischarged or discharged through electrochemical reactions of the activematerials that occur inside the two half-cells. As used herein, theterms “active material,” “electroactive material,” “redox-activematerial” or variants thereof synonymously refer to materials thatundergo a change in oxidation state during operation of a flow batteryor like electrochemical energy storage system (i.e., during charging ordischarging). Although flow batteries hold significant promise forlarge-scale energy storage applications, they have often been plagued bysub-optimal energy storage performance (e.g., round trip energyefficiency) and limited cycle life, among other factors. Despitesignificant investigational efforts, no commercially viable flow batterytechnologies have yet been developed.

The operating performance of flow batteries can be impacted by a numberof factors including, for example, state of charge (SOC), operatingtemperature, age of the flow battery and its components, electrolytecirculation rates, power and current conditions, and the like. As usedherein, the term “state of charge” (SOC) refers to the relative amountsof reduced and oxidized active material species at an electrode within agiven half-cell of a flow battery or other electrochemical system at aparticular operation time. In many cases, the foregoing factors are notindependent of one another, which can make performance optimization verydifficult. Effective regulation of circulation rates throughout a flowbattery is one particular factor that has been especially problematic toaddress and has contributed to their present lack of commercialviability.

In view of the foregoing, flow batteries and associated methodsconfigured to promote more effective circulation of an electrolytesolution would be highly desirable in the art. The present disclosuresatisfies the foregoing needs and provides related advantages as well.

SUMMARY

In various embodiments, the present disclosure provides flow batteriesincluding: an electrochemical cell stack containing a plurality ofelectrochemical cells in electrical communication with one another; aninlet manifold containing an inflow channel fluidically connected to aninflow side of each of the electrochemical cells; an outlet manifoldcontaining an outflow channel fluidically connected to an outflow sideof each of the electrochemical cells; and an insert disposed in theoutflow channel. The insert has a variable width along a length of theoutflow channel.

In other various embodiments, methods for operating a flow battery arealso disclosed herein. The methods can include: providing a flow batterycontaining: an electrochemical cell stack containing a plurality ofelectrochemical cells in electrical communication with one another; aninlet manifold containing an inflow channel fluidically connected to aninflow side of each of the electrochemical cells; and an outlet manifoldcontaining an outflow channel fluidically connected to an outflow sideof each of the electrochemical cells; placing an insert in the outflowchannel; and circulating an electrolyte solution through theelectrochemical cell stack via the inlet manifold and the outletmanifold. The insert has a variable width along a length of the outflowchannel.

The foregoing has outlined rather broadly the features of the presentdisclosure in order that the detailed description that follows can bebetter understood. Additional features and advantages of the disclosurewill be described hereinafter. These and other advantages and featureswill become more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1 depicts a schematic of an illustrative flow battery containing asingle electrochemical cell;

FIG. 2 shows a schematic of an illustrative electrochemical cellcontaining a bipolar plate abutting each electrode;

FIG. 3 shows an illustrative schematic of a bipolar plate containinginterdigitated flow channels;

FIG. 4 shows a generalized schematic of an illustrative electrochemicalcell stack containing the electrochemical cell of FIG. 2;

FIG. 5 shows a generalized schematic of an illustrative electrochemicalcell stack having shared bipolar plates between adjacent electrochemicalcells;

FIG. 6 shows an illustrative schematic of a flow battery containing anelectrochemical cell stack with inlet and outlet manifolds that arefluidically connected to alternating half-cells;

FIG. 7 shows an exploded view of an illustrative electrochemical cellfabricated from mass-produced components;

FIG. 8 shows an illustrative schematic of a flow battery containing anelectrochemical cell stack with inlet and outlet manifolds that arefluidically connected to alternating half-cells and in which a variablewidth insert is disposed in the outlet manifold;

FIG. 9 shows an illustrative schematic of an illustrativeelectrochemical cell stack containing inserts affixed to pipe flanges;

FIG. 10 shows a graph illustrating modeling of circulation rates throughthe individual electrochemical cells of an electrochemical cell stackwhen an insert is present versus when it is not; and

FIG. 11 shows a graph illustrating experimental discharge voltages forindividual electrochemical cells within an electrochemical cell stack asa function of position, in which an insert is not present.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to flow batteriesconfigured for regulating electrolyte solution circulation through anelectrochemical cell stack. The present disclosure is also directed, inpart, to methods for regulating electrolyte solution circulation throughan electrochemical cell stack.

The present disclosure may be understood more readily by reference tothe following description taken in connection with the accompanyingfigures and examples, all of which form a part of this disclosure. It isto be understood that this disclosure is not limited to the specificproducts, methods, conditions or parameters described and/or shownherein. Further, the terminology used herein is for purposes ofdescribing particular embodiments by way of example only and is notintended to be limiting unless otherwise specified. Similarly, unlessspecifically stated otherwise, any description herein directed to acomposition is intended to refer to both solid and liquid versions ofthe composition, including solutions and electrolytes containing thecomposition, and electrochemical cells, flow batteries, and other energystorage systems containing such solutions and electrolytes. Further, itis to be recognized that where the disclosure herein describes anelectrochemical cell, flow battery, or other energy storage system, itis to be appreciated that methods for operating the electrochemicalcell, flow battery, or other energy storage system are also implicitlydescribed.

It is also to be appreciated that certain features of the presentdisclosure may be described herein in the context of separateembodiments for clarity purposes, but may also be provided incombination with one another in a single embodiment. That is, unlessobviously incompatible or specifically excluded, each individualembodiment is deemed to be combinable with any other embodiment(s) andthe combination is considered to represent another distinct embodiment.Conversely, various features of the present disclosure that aredescribed in the context of a single embodiment for brevity's sake mayalso be provided separately or in any sub-combination. Finally, while aparticular embodiment may be described as part of a series of steps orpart of a more general structure, each step or sub-structure may also beconsidered an independent embodiment in itself.

Unless stated otherwise, it is to be understood that each individualelement in a list and every combination of individual elements in thatlist is to be interpreted as a distinct embodiment. For example, a listof embodiments presented as “A, B, or C” is to be interpreted asincluding the embodiments “A,” “B,” “C,” “A or B,” “A or C,” “B or C,”or “A, B, or C.”

In the present disclosure, the singular forms of the articles “a,” “an,”and “the” also include the corresponding plural references, andreference to a particular numerical value includes at least thatparticular value, unless the context clearly indicates otherwise. Thus,for example, reference to “a material” is a reference to at least one ofsuch materials and equivalents thereof.

In general, use of the term “about” indicates approximations that canvary depending on the desired properties sought to be obtained by thedisclosed subject matter and is to be interpreted in a context-dependentmanner based on functionality. Accordingly, one having ordinary skill inthe art will be able to interpret a degree of variance on a case-by-casebasis. In some instances, the number of significant figures used whenexpressing a particular value may be a representative technique ofdetermining the variance permitted by the term “about.” In other cases,the gradations in a series of values may be used to determine the rangeof variance permitted by the term “about.” Further, all ranges in thepresent disclosure are inclusive and combinable, and references tovalues stated in ranges include every value within that range.

As discussed above, energy storage systems that are operable on a largescale while maintaining high efficiency values can be extremelydesirable. Flow batteries have generated significant interest in thisregard, but there remains considerable room for improving theiroperating performance. Exemplary description of illustrative flowbatteries, their use, and operating characteristics, as well as ways inwhich the flow battery structure and operating conditions can bemodified to improve performance, are provided hereinbelow.

A number of parameters can impact the operating performance of a givenflow battery, and in many cases these parameters are interdependent uponone another. The circulation rates of the electrolyte solutions in aflow battery can particularly impact operating performance. Ineffectivecirculation rates, either too fast or too slow, can lead to poor energyefficiency values, for example. Present flow battery designs cancontribute to a difficulty in regulating circulation rates of theelectrolyte solutions, as explained hereinafter.

Flow batteries normally contain a plurality of electrochemical cellsdisposed in an electrochemical cell stack (i.e., a vertically disposedgrouping of abutted electrochemical cells, each containing opposinghalf-cells divided by a separator—see FIGS. 4 and 5). With present flowbattery designs, it can be difficult, if not impossible, to regulateelectrolyte solution circulation rates through each electrochemical cellindividually. In particular, the electrochemical cells in a verticallydisposed electrochemical cell stack experience different back pressureswhen circulating an electrolyte solution therethrough, thereby leadingto different circulation rates in a given electrochemical cell dependingupon how much back pressure is present at a given location. In general,the electrochemical cells nearer the location of electrolyte solutionintroduction and withdrawal experience a different (smaller) pressuredrop on their outlet end than do the electrochemical cells that are moreremote from the locations of introduction and withdrawal. The variablepressure drop results in differing electrolyte solution circulationrates throughout the individual electrochemical cells of anelectrochemical cell stack. Aging or failing components of a flowbattery can similarly impact electrolyte solution circulation rates insome cases. As a result of the lack of circulation uniformity, at leastsome of the electrochemical cells in an electrochemical cell stack canbe operating at a less than optimal condition.

The present inventors discovered that by adjusting the back pressure atan outlet end of each electrochemical cell within an electrochemicalcell stack, the electrolyte solution circulation rate(s) through theindividual electrochemical cells can be made more uniform. Morespecifically, the inventors discovered that by variably occluding anoutflow channel through which the electrolyte solution passes, the backpressure and the electrolyte solution circulation rate can be made moreuniform at each electrochemical cell. In the discovery made by theinventors, an insert can be placed in the outflow channel to providemore flow occlusion for some electrochemical cells and less flowocclusion for others. The foregoing can be accomplished by designing theinsert to have a variable width along a length of the outflow channeland placing the insert in the outflow channel. In particular, the inserthas larger widths at locations in the outflow channel that are moreremoved from the location of electrolyte solution withdrawal, where agreater back pressure is needed to levelize the circulation rate, andsmaller widths at locations nearer the location of withdrawal. Morespecifically, the insert can be tapered to have an increasing width fromthe top face to the bottom face of the electrochemical cell stackrelative to the locations of electrolyte solution introduction andwithdrawal. The tapering can be linear or non-linear, and uniform ornon-uniform. Advantageously, these factors can be varied to meet therequirements of a given application.

Flow batteries are most commonly fabricated by combining mass-producedelectrochemical cell components together to form an electrochemical cellstack. Mass production allows multiple electrochemical cells to befabricated relatively inexpensively. Once assembled into anelectrochemical cell stack, the electrochemical cell components definean inflow channel and an outflow channel therein, through which anelectrolyte solution can be circulated to a particular half-cell of eachelectrochemical cell. While fabricating electrochemical cell stacks inthis manner can allow inexpensive production to take place, it does notallow customization of any feature to be readily realized. Althoughcustom-designed inflow and/or outflow channels could be fabricated toaddress the difficulties associated with differential pressure drops,doing so would considerably impact manufacturing costs and productionrates.

In contrast, by disposing a variable width insert in the outflow channelof an outlet manifold associated with an electrochemical cell, readycustomization of the electrolyte solution circulation performance can bemore easily realized without significantly increasing the manufacturingcomplexity and cost. More particularly, the electrochemical cell stackcan be fabricated using low-cost, mass-produced electrochemical cellcomponents, and depending upon the pressure profile that is predicted orobserved, an insert with a desired design or profile can be selected tomodify the pressure features in the outflow channel in a specific way.Even custom-designed inserts with complex shapes do not addsignificantly to the overall cost once averaged with the mass-producedcomponents.

While the cell components used to fabricate the electrochemical cellstack can be mass-produced, the insert can be made by more customizedmanufacturing processes, if needed. The insert can be a monolithicstructure, or the insert can be assembled from a plurality of modularcomponents to produce an insert having a desired shape. The latterconfiguration allows a great deal of design flexibility to be realized.In some embodiments, the modular components can be cheaply produced toprovide a “toolkit” that is capable of forming an insert having a widevariety of usable shapes. Techniques for affixing the modular componentstogether to form the inset are not considered to be particularlylimited.

As an additional benefit of the present disclosure, the insert canfurther modify the circulation performance in the outflow channel in adesired way. For example, the insert can, in some embodiments, beconfigured to induce or reduce turbulence in the outflow channel,restrict circulation rates in the outflow channel, or the like.

Moreover, in many instances, the insert can be removably disposed withinthe outflow channel of the electrochemical cell stack. With the insertbeing disposed in a removable condition, the operating performance ofthe flow battery can be evaluated, and the insert can be replaced with adifferent insert that is capable of inducing one or more differing flowand/or pressure conditions within the outflow channel, if desired. Thisprocess can be repeated as many times as necessary to find a set ofdesired circulation and/or pressure conditions. In addition, if thecirculation pathway within the electrochemical cells changes over time(e.g., by becoming partially clogged with deposited active material orother components, thereby increasing the back pressure), the insert canbe changed in response to the new pressure conditions.

As still another advantage, the insert can be affixed to a componentconfigured to mate with an outlet of the outflow channel, therebydisposing the insert within the outflow channel upon connecting thecomponent to the outlet. In particular embodiments, the component can bea flange that is operably connected to the outlet, although any suitablecomponent not associated with the electrochemical cell stack can be usedto introduce the insert within the outflow channel. As used herein, theterm “balance of plant (BOP)” refers to any of the system components ofa flow battery that are outside the electrochemical cell stack. Byaffixing the insert to a flange or other balance of plant component,movement of the insert within the outflow channel can be precluded. Inalternative configurations, however, the insert can be affixed withinthe outlet change through other means, such as through adhesive bonding.

In addition to the foregoing advantages, the insert can be design tomitigate shunt currents within the flow battery system. For example, bydecreasing a fluidic cross-section within the outflow channel, the shuntresistance can undergo a corresponding increase. Similarly, by creatingisolated flow pathways between individual cells, some of which arelonger, shunt currents can be decreased.

Before discussing further specifics of the flow batteries and methods ofthe present disclosure, illustrative flow battery configurations andtheir operating characteristics will first be described in greaterdetail hereinafter.

FIG. 1 depicts a schematic of an illustrative flow battery containing asingle electrochemical cell. Although FIG. 1 shows a flow batterycontaining a single electrochemical cell, approaches for combiningmultiple electrochemical cells together in an electrochemical cell stackare known and are discussed hereinbelow. Unlike typical batterytechnologies (e.g., Li-ion, Ni-metal hydride, lead-acid, and the like),where active materials and other components are housed in a singleassembly, flow batteries transport (e.g., via pumping) redox-activeenergy storage materials from storage tanks through an electrochemicalcell stack. This design feature decouples the electrical energy storagesystem power from the energy storage capacity, thereby allowing forconsiderable design flexibility and cost optimization to be realized.

As shown in FIG. 1, flow battery 1 includes an electrochemical cell thatfeatures separator 20 (e.g., a membrane) that separates the twoelectrodes 10 and 10′ of the electrochemical cell. As used herein, theterms “separator” and “membrane” synonymously refer to an ionicallyconductive and electrically insulating material disposed between thepositive and negative electrodes of an electrochemical cell. Electrodes10 and 10′ are formed from a suitably conductive material, such as ametal, carbon, graphite, and the like. Although FIG. 1 has shownelectrodes 10 and 10′ as being spaced apart from separator 20,electrodes 10 and 10′ can also be abutted with separator 20 in moreparticular embodiments. The material(s) forming electrodes 10 and 10′can be porous, such that they have a high surface area for contactingfirst electrolyte solution 30 and second electrolyte solution 40, theactive materials of which are capable of cycling between an oxidizedstate and a reduced state during operation of flow battery 1. Forexample, one or both of electrodes 10 and 10′ can be formed from aporous carbon cloth or a carbon foam in particular embodiments.

Pump 60 affects transport of first electrolyte solution 30 containing afirst active material from tank 50 to the electrochemical cell. The flowbattery also suitably includes second tank 50′ that holds secondelectrolyte solution 40 containing a second active material. The secondactive material in second electrolyte solution 40 can be the samematerial as the first active material in first electrolyte solution 30,or it can be different. Second pump 60′ can affect transport of secondelectrolyte solution 40 to the electrochemical cell. Pumps (not shown inFIG. 1) can also be used to affect transport of the first and secondelectrolyte solutions 30 and 40 from the electrochemical cell back totanks 50 and 50′. Other methods of affecting fluid transport, such assiphons, for example, can also suitably transport first and secondelectrolyte solutions 30 and 40 into and out of the electrochemicalcell. Also shown in FIG. 1 is power source or load 70, which completesthe circuit of the electrochemical cell and allows a user to collect orstore electricity during its operation. Connection to the electricalgrid for charging or discharging purposes can also occur at thislocation.

It should be understood that FIG. 1 depicts a specific, non-limitingembodiment of a flow battery. Accordingly, flow batteries consistentwith the spirit of the present disclosure can differ in various aspectsrelative to the configuration of FIG. 1. As one example, a flow batterycan include one or more active materials that are solids, gases, and/orgases dissolved in liquids. Active materials can be stored in a tank, ina vessel open to the atmosphere, or simply vented to the atmosphere.

During operation of a flow battery in a charging cycle, one of theactive materials undergoes oxidation and the other active materialundergoes reduction. In a discharging cycle, the opposite processesoccur in each half-cell. Upon changing the oxidation states of theactive materials, the chemical potentials of the electrolyte solutionsare no longer in balance with one another. To relieve the chemicalpotential imbalance, dissolved mobile ions migrate through the separatorto lower the charge in one electrolyte solution and to raise the chargein the other electrolyte solution. Thus, the mobile ions transfer thecharge generated upon oxidizing or reducing the active materials, butthe mobile ions themselves are not usually oxidized or reduced. Tomaintain facile electrode kinetics and to limit the possibility ofoccluding flow pathways, flow batteries are usually configured such thatthe mobile ions and the active materials remain continuously dissolvedin the electrolyte solutions. By keeping the mobile ions and the activematerials continuously dissolved in the electrolyte solutions, potentialissues associated with circulating solids can be averted.

As indicated above, multiple electrochemical cells can also be combinedwith one another in an electrochemical cell stack in order to increasethe rate that energy can be stored and released during operation. Theamount of energy released is determined by the overall amounts of activematerials that are present. An electrochemical cell stack utilizesbipolar plates between adjacent electrochemical cells to establishelectrical communication but not fluidic communication between the twocells across the bipolar plate. Thus, bipolar plates contain theelectrolyte solutions in an appropriate half-cell within the individualelectrochemical cells. Bipolar plates are generally fabricated fromelectrically conductive materials that are fluidically non-conductive onthe whole. Suitable materials can include carbon, graphite, metal, or acombination thereof. Bipolar plates can also be fabricated fromnon-conducting polymers having a conductive material dispersed therein,such as carbon particles or fibers, metal particles or fibers, graphene,and/or carbon nanotubes. Although bipolar plates can be fabricated fromthe same types of conductive materials as can the electrodes of anelectrochemical cell, they can lack the continuous porosity permittingan electrolyte solution to flow completely through the latter. It shouldbe recognized that bipolar plates are not necessarily entirelynon-porous entities, however. Bipolar plates can have innate or designedflow channels, for example, that provide a greater surface area forallowing an electrolyte solution to contact the bipolar plate. Suitableflow channel configurations can include, for example, interdigitatedflow channels (see FIG. 3). In some embodiments, the flow channels canbe used to promote delivery of an electrolyte solution to an electrodewithin the electrochemical cell. Delivery of an electrolyte solution toan electrode via a bipolar plate is discussed in more detailhereinbelow.

An electrolyte solution can be delivered to and withdrawn from eachelectrochemical cell via an inlet manifold and an outlet manifold (notshown in FIG. 1). In some embodiments, the inlet manifold and the outletmanifold can provide and withdraw an electrolyte solution via thebipolar plates separating adjacent electrochemical cells. Separate inletmanifolds can provide each electrolyte solution individually to the twohalf-cells of each electrochemical cell. Likewise, separate outletmanifolds withdraw the electrolyte solutions from the positive andnegative half-cells. In more particular embodiments, the inlet manifoldand the outlet manifold can be configured to supply and withdraw theelectrolyte solutions via opposing lateral faces of the bipolar plates(e.g. by supplying and withdrawing the electrolyte solution fromopposing ends of the flow channels within the bipolar plate). Thus, theelectrolyte solutions circulate laterally through the individualhalf-cells of the flow battery. Further disclosure is providedhereinbelow regarding how the inlet and outlet manifolds are configured,and particular disclosure is provided showing how the outlet manifoldcan be modified using an insert.

FIG. 2 shows a schematic of an illustrative electrochemical cellcontaining a bipolar plate abutting each electrode. Where appropriate,common reference characters are used herein to describe elements shownin a preceding FIGURE. Referring to FIG. 2, negative half-cell 80 andpositive half-cell 80′ are disposed on opposing sides of separator 20.Negative half-cell 80 contains electrode 10 (i.e., the anode) abuttedwith separator 20 at interface 12, and bipolar plate 90 is, in turn,abutted against the opposing face of electrode 10 at interface 14.Positive half-cell 80′ similarly contains electrode 10′ (i.e., thecathode) abutted with the opposing face of separator 20 at interface12′, and bipolar plate 90′ is, in turn, abutted against the opposingface of electrode 10′ at interface 14′. Flow channels 82 extendpartially within the interior of bipolar plates 90 and 90′ and increasethe degree of contact with the electrolyte solution. In someembodiments, flow channels 82 can be in an interdigitated configurationas shown in FIG. 3 below. Other configurations for flow channelsinclude, for example, regular or irregular spacing, randomdirectionality, tortuous interconnected pathways, random distributionsand/or gradient distributions. In the interest of clarity, the fluidflow details shown in FIG. 1 are not presented in FIG. 2. However, itcan be readily appreciated how the electrochemical cell of FIG. 2 wouldbe incorporated flow battery 1 in FIG. 1, or how a plurality ofelectrochemical cells would be incorporated in an electrochemical cellstack and connected to inlet and outlet manifolds to deliver andwithdraw an electrolyte solution. For example, an inlet manifold can beconnected to an inlet side on bipolar plates 90 and 90′ to supply anelectrolyte solution to electrodes 10 and 10′, as shown hereinafter. Forpurposes of discussion herein, the electrochemical cell of FIG. 2 willbe considered representative of that present in a conventional flowbattery.

FIG. 3 shows an illustrative schematic of a bipolar plate containinginterdigitated flow channels. As shown in FIG. 3, bipolar plate 90includes inflow channel 91 as part of an inlet manifold which provideselectrolyte solution to bipolar plate 90, and outflow channel 92 as partof an outlet manifold which withdraws electrolyte solution from bipolarplate 90. Flow channels 82, which are interdigitated with one another inFIG. 3, are in fluidic communication with inflow channel 91 and outflowchannel 92 and supply electrolyte solution to the electrode (not shownin FIG. 3) in a given half-cell. Thus, inflow channel 91 can supply anelectrolyte solution to alternating flow channels 82. After interactingwith the electrode, the electrolyte solution can then migrate viaconvective flow to flow channels 82 beside those that were initiallyfilled with the electrolyte solution. At this point, the electrolytesolution can then exit bipolar plate 90 via outflow channel 92.

Accordingly, in various embodiments, flow batteries of the presentdisclosure can include: an electrochemical cell stack containing aplurality of electrochemical cells in electrical communication with oneanother, an inlet manifold containing an inflow channel fluidicallyconnected to an inflow side of each of the electrochemical cells, anoutlet manifold containing an outflow channel fluidically connected toan outflow side of each of the electrochemical cells, and an insertdisposed in the outflow channel. The insert has a variable width along alength of the outflow channel. In more specific embodiments, a width ofthe insert decreases in a direction configured for circulation of theelectrolyte solution in the outflow channel.

FIG. 4 shows a generalized schematic of an illustrative electrochemicalcell stack containing the electrochemical cell of FIG. 2. As shown inFIG. 4, electrochemical cells 80 a, 80 b and 80 c are abutted againstone another in electrochemical cell stack 84, such that electricalcommunication is established through their abutted bipolar plates. Inalternative configurations, electrochemical cell stacks can also beformed such that they share bipolar plates between adjacentelectrochemical cells. FIG. 5 shows a generalized schematic of anillustrative electrochemical cell stack 90 having shared bipolar platesbetween adjacent electrochemical cells 92 a, 92 b and 92 c. Although theunit cell structure of FIG. 5 differs somewhat from that of FIG. 4, onehaving ordinary skill in the art can envision how such anelectrochemical cell stack can be fabricated by sequentially placingmass-produced components upon one another. In either case, stacking ofthe mass-produced components produces inlet and outlet manifolds fordelivering and withdrawing electrolyte solution, as discussed furtherhereinafter.

As discussed above, each electrochemical cell of a flow battery includesa negative half-cell and a positive half-cell, which are on opposingsides of an ionically conductive separator or membrane. Each half-cellcan have separate inlet and outlet manifolds fluidically connectedthereto. That is, in an electrochemical cell stack, a first inletmanifold and a first outlet manifold can supply and withdraw a firstelectrolyte solution to and from the negative half-cells of theelectrochemical cell stack, and a second inlet manifold and a secondoutlet manifold can supply and withdraw a second electrolyte solution toand from the positive half-cells of the electrochemical cell stack. Inthe embodiments of the present disclosure, the first outlet manifold,the second outlet manifold, or both the first and second outletmanifolds can have an insert present therein in order to regulateelectrolyte solution circulation through the corresponding half-cells.

In more particular embodiments, the inflow channel is configured tosupply electrolyte solution through the inlet manifold longitudinallywith respect to the plurality of electrochemical cells, the outflowchannel is configured to withdraw electrolyte solution though the outletmanifold longitudinally with respect to the plurality of electrochemicalcells, and each electrochemical cell is configured to circulateelectrolyte solution laterally therethrough. As used herein, the term“longitudinally” refers to a direction that is substantiallyperpendicular (±10°, for example) to the plane of the individualelectrochemical cells in an electrochemical cell stack. As used herein,the term “laterally” refers to a direction that is substantiallyin-plane within the individual electrochemical cells of anelectrochemical cell stack. In still more particular embodiments, theinlet manifold and the outlet manifold are configured, respectively, tosupply and withdraw electrolyte solution longitudinally from a singleface of the electrochemical cell stack.

FIG. 6 shows an illustrative schematic of a flow battery containing anelectrochemical cell stack with inlet and outlet manifolds that arefluidically connected to alternating half-cells. Since FIG. 6 lacks aninsert in the outflow channel, the flow battery configuration of FIG. 6is considered to be representative of that of a conventional flowbattery. In the interest of clarity, the inlet and outlet manifolds thatare fluidically connected to the oppositely charged half-cells of theflow battery are not depicted. Similarly, FIG. 6 omits some of thestructural details shown in the preceding FIGURES so that variousconfigurations of the inlet and outlet manifolds can be betterunderstood. For example, the block arrows in FIG. 6 show electrolytesolution circulating laterally through the half-cells, but it is to berecognized that, in more specific embodiments, the electrolyte solutioncan be introduced by way of the bipolar plate to accomplish such lateralcirculation (see FIG. 3, for example). Furthermore, although FIG. 6shows a single bipolar plate separating the individual electrochemicalcells in the electrochemical cell stack, such as the configuration shownin FIG. 5, it is to be recognized that abutted bipolar plates, such asthe configuration of FIG. 4, can be used similarly.

As shown in FIG. 6, flow battery 100 includes electrochemical cell stack102, which contains a plurality of electrochemical cells that areseparated by one or more bipolar plates (see FIGS. 4 and 5). Since noinsert is present in FIG. 6, flow battery 100 contains no provisions forback pressure regulation in outlet manifold 106. Inlet manifold 104 andoutlet manifold 106 respectively supply and withdraw electrolytesolution to and from either the positive or negative half-cells withinelectrochemical cell stack 102. Inlet manifold 104 contains inflowchannel 105, and outlet manifold 106 contains outflow channel 107. Asdepicted in block arrows in FIG. 6, an electrolyte solution isintroduced into inlet manifold 104 longitudinally with respect to theelectrochemical cells of electrochemical cell stack 102 and is alsowithdrawn longitudinally from outlet manifold 106. Similarly, lateralcirculation of the electrolyte solution through one type of half-cellwithin each electrochemical cell of electrochemical cell stack 102 isshown in block arrows in FIG. 6. In the interest of clarity, circulationof an electrolyte solution through the other type of half-cell withinflow battery 100 is not shown but is understood to be present. Asindicated above, the electrolyte solution can enter the given half-cellsvia flow channels in the bipolar plates, according to some embodiments(see FIG. 3).

In some embodiments, inlet manifold 104 and its associated inflowchannel 105 and outlet manifold 106 and its associated outflow channel107 can be contiguous with the components used to fabricate the variouselectrochemical cells of electrochemical cell stack 102. As explainedabove, various electrochemical cells can be fabricated usingmass-produced components, which also serve to define a flow pathway forthe electrolyte solution. As electrochemical cell stack 102 isfabricated by stacking the mass-produced components upon one another,inflow channel 105 and outflow channel 107 become defined adjacent tothe electrically conductive elements of electrochemical cell stack 102.Commonly owned U.S. patent application Ser. No. 15/093,598, filed onApr. 7, 2016 and incorporated herein by reference in its entirety,provides additional details concerning how an electrochemical cell stackcan be fabricated from mass-produced components, as explainedhereinafter.

FIG. 7 shows an exploded view of an illustrative electrochemical cell120 fabricated from mass-produced components. In brief, electrochemicalcell 120 includes electrodes 124 and 124′ disposed on opposing sides ofseparator 122, and bipolar plates 126 and 126′ adjacent to electrodes124 and 124′, respectively. Various frame layers 130 a-d and 130 a′-d′promote distribution of electrolyte solution to bipolar plates 126 and126′ and electrodes 124 and 124′ upon combining the layers together todefine electrochemical cell 120. A plurality of electrochemical cells120 can be combined to form an electrochemical cell stack (see FIGS. 3and 4, for example). Frame layers 130 a-d and 130 a′-d′ containapertures 140 a, 140 b, 141 a and 141 b, which ultimately define theinflow and outflow channels that are in fluidic communication with eachelectrochemical cell. Specifically, a plurality of apertures 140 a and141 a define an inflow channel and an outflow channel for one type ofhalf-cell, and a plurality of apertures 140 b and 141 b define an inflowchannel and an outflow channel for the other type of half-cell. As such,the resulting inflow and outflow channels are configured to promotelateral circulation through the electrochemical cells separately withineach half-cell.

Referring again to FIG. 6, and as discussed further above, each of theelectrochemical cells within electrochemical cell stack 102 experiencedifferent back pressures, which contribute to differential circulationrates in each electrochemical cell. Specifically, the electrochemicalcell at the bottom of outflow channel 107 (i.e., the electrochemicalcell most removed from the point of electrolyte solution withdrawal fromoutlet manifold 106) experiences a different back pressure than does theelectrochemical cell at the top. In between, the back pressure variesbetween these two extremes. In addition, the electrolyte solutionpressure on the inflow side of the electrochemical cells increases withincreasing depth within inflow channel 106. In combination, theresulting pressure imbalance leads to different circulation rates of theelectrolyte solution through each of the electrochemical cells.

As indicated above, the present inventors discovered a ready techniqueto combat this type of pressure imbalance, even when mass-produced cellcomponents are used to fabricate repeating electrochemical cells of likestructure in an electrochemical cell stack. Specifically, the presentinventors discovered that placing an insert within an outflow channel ofthe electrochemical cell stack can variably regulate the back pressureexperienced by an electrochemical cell in a particular longitudinallocation. In order to produce a back pressure that varies longitudinallywithin the electrochemical cell stack, an insert having a variable widthalong a length of the outflow channel can be employed. In particular,the insert can be tapered along the length of the outflow channel, andstructural aspects of the tapering can be further tailored to promoteparticular flow conditions in the outflow channel. In general, the widthof the insert decreases in a direction configured for circulation of theelectrolyte solution in the outflow channel.

FIG. 8 shows an illustrative schematic of a flow battery containing anelectrochemical cell stack with inlet and outlet manifolds that arefluidically connected to alternating half-cells and in which a variablewidth insert is disposed in the outlet manifold. Since the flow batteryconfiguration of FIG. 8 is substantially similar to that of FIG. 6,except for the present of the variable width insert, common referencecharacters are used in these FIGURES to describe elements having similarstructure and/or function. The variable width insert alters the pressuredrop occurring through the height of the electrochemical cell stack,thereby levelizing the circulation rate of the electrolyte solutionthroughout the plurality of electrochemical cells.

Referring to FIG. 8, flow battery 150 includes electrochemical cellstack 102, inlet manifold 104 and outlet manifold 106. Within outflowchannel 107 of outlet manifold 106 is disposed insert 160. Insert 160 iswider at locations that are farther removed from an outlet of outflowchannel 107 (i.e., at the top of inlet manifold 106), thereby decreasingthe area at the bottom of outflow channel 107 and increasing the backpressure exerted there. Conversely, since insert 160 decreases in areaas it approaches the top of outflow channel 107, a smaller increase inback pressure occurs at this location under the influence of insert 160.The pitch of the tapering can be adjusted to promote a desired backpressure at each longitudinal position within outflow channel 107.Accordingly, the pitch and insert configuration depicted in FIG. 8should not be considered limiting. For example, in alternativeembodiments, insert 160 can have a curved surface, such as a parabolicor hyperbolic surface. In addition, insert 160 need not necessarilyexhibit as constant change in width along the length of outflow channel107, as depicted. For example, in some embodiments, a stepwise change inwidth can be effective, and in still other embodiments, a first portionof the insert can be curved and a second portion of the insert can belinearly tapered.

In some embodiments, the insert can be a monolithic structure.Monolithic structures can oftentimes be manufactured in a relativelystraightforward manner. Suitable manufacturing techniques for amonolithic insert can include milling, casting, 3-D printing and thelike. In addition, monolithic structures can usually be introduced tothe outflow channel rather easily.

In other embodiments, the insert can be assembled from a plurality ofmodular components, which are smaller in size than the insert. Assemblyof the insert from modular components can be desirable, for example, forprototype testing before fabricating a more robust insert as amonolithic structure. In addition, assembly of the insert from aplurality of modular components can be desirable if the insert has acomplicated structure that may be problematic to fabricate by typicalmanufacturing means. The modular components can be in the form of a“toolkit” that can enable production of a wide variety of inserts havinga range of shapes.

In general, materials used for fabricating the inserts are notconsidered to be particularly limited. Suitable materials can include,for example, polymers, ceramics, glass, metals and carbon. Polymers canbe especially desirable, since they are commonly used to fabricate avariety of components within the electrochemical cells of theelectrochemical cell stack.

Still other optional features can also be present in variousconfigurations of the insert. In various embodiments, suitable featuresthat can also be incorporated on or within the insert can include, forexample, turbulence inducers, a plurality of projecting hairs, flowchannels, baffles, fins, and the like. Such optional features can alsopromote regulation of the electrolyte solution circulation rate in theoutflow channel in a desired and configurable manner.

In some embodiments, the insert can be affixed within the outflowchannel. Suitable attachment techniques for affixing the insert withinthe outflow channel can include, for example, adhesive bonding, meltbonding, welding, mechanical attachment, compression fitting, and thelike. The particular technique chosen for affixing the insert within theoutflow channel can depend upon the material forming the interior of theoutflow channel, for example.

More desirably, however, the insert can be removably disposed within theoutflow channel. By configuring the insert to be removable, much greateroperational flexibility can be realized. For example, if the insertfails or no longer performs as expected, the insert can be replaced by anew insert that is configured differently. Similarly, a new insert canbe employed if the circulation conditions within the components of theflow battery change with age or use. Regardless of the need for making achange within the electrochemical cell stack, a new insert can be placedwithin the outflow channel to facilitate new circulation conditionsthrough the electrochemical cells.

In some embodiments, the insert can be affixed to a component containedwith the balance of plant. Any component that is close enough to theoutflow channel to promote introduction of the insert therein can beused. In particular embodiments, the balance of plant of a particularflow battery include a flange connected to an outlet of the outflowchannel. The flange can help promote making a robust fluidic connectionof a line leading to an electrolyte solution storage tank, for example.In specific embodiments, the insert can be affixed to the flange.Because the flange is located in such close proximity to the outflowchannel, it can be particularly desirable to affix the insert to thiscomponent for disposition within the outflow channel. FIG. 9 shows anillustrative schematic of an illustrative electrochemical cell stack 200containing inserts 210 affixed to flanges 220. Although FIG. 9 has showninserts 210 within both the inflow channel and the outflow channel ofelectrochemical cell stack 200, it is to be recognized that the inserts210 within the inflow channel can be omitted, so as to be consistentwith the disclosure above. Further FIG. 9 shows the placement ofseparate inserts 210 for each of the electrolyte solutions circulatingthrough electrochemical cell stack 200.

Each of the half-cells in the flow batteries of the present disclosureinclude an electrode. In more particular embodiments, one or both of theelectrodes in the half-cells can be a carbon electrode, which can beformed from a carbon cloth or a carbon foam in some instances. Numerousexamples of carbon cloths or carbon foams suitable for forming a carbonelectrode will be familiar to one having ordinary skill in the art.

In some embodiments, flow batteries of the present disclosure caninclude an active material that is a coordination complex in one or moreof the electrolyte solutions. As used herein, the terms “coordinationcomplex,” “coordination compound” and similar terms refer to anycompound having a metal bound to one or more ligands through a covalentbond. Due to their variable oxidation states, transition metals can behighly desirable for use within the active materials of a flow batterysystem. Lanthanide metals can be used similarly in alternativeembodiments. Cycling between the accessible oxidation states can resultin the conversion of chemical energy into electrical energy. Especiallydesirable transition metals for inclusion in a flow battery include, forexample, Al, Cr, Ti and Fe, particularly in the form of a coordinationcomplex. For purposes of the present disclosure, Al is to be considereda transition metal. In some embodiments, coordination complexes within aflow battery can include at least one catecholate or substitutedcatecholate ligand.

Other ligands that can be present in coordination complexes, alone or incombination with one or more catecholate or substituted catecholateligands, include, for example, ascorbate, citrate, glycolate, a polyol,gluconate, hydroxyalkanoate, acetate, formate, benzoate, malate,maleate, phthalate, sarcosinate, salicylate, oxalate, urea, polyamine,aminophenolate, acetylacetonate, and lactate. Where chemically feasible,it is to be recognized that such ligands can be optionally substitutedwith at least one group selected from among C₁₋₆ alkoxy, C₁₋₆ alkyl,C₁₋₆ alkenyl, C₁₋₆ alkynyl, 5- or 6-membered aryl or heteroaryl groups,a boronic acid or a derivative thereof, a carboxylic acid or aderivative thereof, cyano, halide, hydroxyl, nitro, sulfonate, asulfonic acid or a derivative thereof, a phosphonate, a phosphonic acidor a derivative thereof, or a glycol, such as polyethylene glycol.Alkanoate includes any of the alpha, beta, and gamma forms of theseligands. Polyamines include, but are not limited to, ethylenediamine,ethylenediamine tetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA).

Other examples of ligands can be present include monodentate, bidentate,and/or tridentate ligands. Examples of monodentate ligands that can bepresent in a coordination complex include, for example, carbonyl orcarbon monoxide, nitride, oxo, hydroxo, water, sulfide, thiols,pyridine, pyrazine, and the like. Examples of bidentate ligands that canbe present in a coordination complex include, for example, bipyridine,bipyrazine, ethylenediamine, diols (including ethylene glycol), and thelike. Examples of tridentate ligands that can be present a coordinationcomplex include, for example, terpyridine, diethylenetriamine,triazacyclononane, tris(hydroxymethyl)aminomethane, and the like.

In some embodiments, one or more of the active materials can becoordination complexes having a formula ofD _(g) M(L ₁)(L ₂)(L ₃),wherein D is an alkali metal ion, an ammonium ion, a tetraalkylammoniumion, a phosphonium ion or any combination thereof, g is an integer ornon-integer value ranging between 1 and 6, M is a transition metal orlanthanide metal, and L₁-L₃ are bidentate ligands, such as those definedhereinabove. The value of g can depend upon whether L₁-L₃ bear an ioniccharge. In some embodiments, at least one of L₁-L₃ can be a catecholateligand or substituted catecholate ligand, and in other embodiments, eachof L₁-L₃ is a catecholate ligand or a substituted catecholate ligand. Insome or other embodiments, M is Ti. In embodiments in which M is Ti andL₁-L₃ are uncharged catecholate ligands, g has a value of 2 to providecharge balance against titanium (IV).

In more particular embodiments, flow batteries of the present disclosurecan include one or more aqueous electrolyte solutions. As used herein,the term “aqueous electrolyte solution” refers to a homogeneous liquidphase with water as a predominant solvent in which an active material isat least partially solubilized, ideally fully solubilized. Thisdefinition encompasses both solutions in water and solutions containinga water-miscible organic solvent as a minority component of an aqueousphase.

Illustrative water-miscible organic solvents that can be present in anaqueous electrolyte solution include, for example, alcohols and glycols,optionally in the presence of one or more surfactants or othercomponents discussed below. In more specific embodiments, an aqueouselectrolyte solution can contain at least about 98% water by weight. Inother more specific embodiments, an aqueous electrolyte solution cancontain at least about 55% water by weight, or at least about 60% waterby weight, or at least about 65% water by weight, or at least about 70%water by weight, or at least about 75% water by weight, or at leastabout 80% water by weight, or at least about 85% water by weight, or atleast about 90% water by weight, or at least about 95% water by weight.In some embodiments, an aqueous electrolyte solution can be free ofwater-miscible organic solvents and consist of water alone as a solvent.

In further embodiments, an aqueous electrolyte solution can include aviscosity modifier, a wetting agent, or any combination thereof.Suitable viscosity modifiers can include, for example, corn starch, cornsyrup, gelatin, glycerol, guar gum, pectin, and the like. Other suitableexamples will be familiar to one having ordinary skill in the art.Suitable wetting agents can include, for example, various non-ionicsurfactants and/or detergents. In some or other embodiments, an aqueouselectrolyte solution can further include a glycol or a polyol. Suitableglycols can include, for example, ethylene glycol, diethylene glycol,and polyethylene glycol. Suitable polyols can include, for example,glycerol, mannitol, sorbitol, pentaerythritol, andtris(hydroxymethyl)aminomethane. Inclusion of any of these components inan aqueous electrolyte solution can help promote dissolution of acoordination complex or similar active material and/or reduce viscosityof the aqueous electrolyte solution for conveyance through a flowbattery, for example.

In addition to a solvent and a coordination complex as an activematerial, an aqueous electrolyte solution can also include one or moremobile ions (i.e., an extraneous electrolyte). In some embodiments,suitable mobile ions can include proton, hydronium, or hydroxide. Inother various embodiments, mobile ions other than proton, hydronium, orhydroxide can be present, either alone or in combination with proton,hydronium or hydroxide. Such alternative mobile ions can include, forexample, alkali metal or alkaline earth metal cations (e.g., Li⁺, Na⁺,K⁺, Mg²⁺, Ca²⁺ and Sr²⁺) and halides (e.g., F⁻, Cl⁻, or Br⁻). Othersuitable mobile ions can include, for example, ammonium andtetraalkylammonium ions, chalcogenides, phosphate, hydrogen phosphate,phosphonate, nitrate, sulfate, nitrite, sulfite, perchlorate,tetrafluoroborate, hexafluorophosphate, and any combination thereof. Insome embodiments, less than about 50% of the mobile ions can constituteprotons, hydronium, or hydroxide. In other various embodiments, lessthan about 40%, less than about 30%, less than about 20%, less thanabout 10%, less than about 5%, or less than about 2% of the mobile ionscan constitute protons, hydronium, or hydroxide.

Flow batteries of the present disclosure can provide sustained charge ordischarge cycles of several hour durations. As such, they can be used tosmooth energy supply/demand profiles and provide a mechanism forstabilizing intermittent power generation assets (e.g., from renewableenergy sources such as solar and wind energy). It should be appreciated,then, that various embodiments of the present disclosure include energystorage applications where such long charge or discharge durations aredesirable. For example, in non-limiting examples, the flow batteries ofthe present disclosure can be connected to an electrical grid to allowrenewables integration, peak load shifting, grid firming, baseload powergeneration and consumption, energy arbitrage, transmission anddistribution asset deferral, weak grid support, frequency regulation, orany combination thereof. When not connected to an electrical grid, theflow batteries of the present disclosure can be used as power sourcesfor remote camps, forward operating bases, off-grid telecommunications,remote sensors, the like, and any combination thereof.

In some embodiments, flow batteries can include: a first chambercontaining a negative electrode contacting a first aqueous electrolytesolution, a second chamber containing a positive electrode contacting asecond aqueous electrolyte solution, and a separator disposed betweenthe first and second aqueous electrolyte solutions. The chambers provideseparate reservoirs within the cell, through which the first and/orsecond electrolyte solutions circulate so as to contact the respectiveelectrodes and the separator. Each chamber and its associated electrodeand electrolyte solution define a corresponding half-cell. The separatorprovides several functions which include, for example, (1) serving as abarrier to mixing of the first and second electrolyte solutions. (2)electrically insulating to reduce or prevent short circuits between thepositive and negative electrodes, and (3) to facilitate ion transportbetween the positive and negative electrolyte chambers, therebybalancing electron transport during charge and discharge cycles. Thenegative and positive electrodes provide a surface where electrochemicalreactions can take place during charge and discharge cycles. During acharge or discharge cycle, electrolyte solutions can be transported fromseparate storage tanks through the corresponding chambers, as shown inFIG. 1. In a charging cycle, electrical power can be applied to the cellsuch that the active material contained in the second electrolytesolution undergoes a one or more electron oxidation and the activematerial in the first electrolyte solution undergoes a one or moreelectron reduction. Similarly, in a discharge cycle the second activematerial is reduced and the first active material is oxidized togenerate electrical power. As discussed hereinabove, adjustment of thecirculation rates can promote optimization of this process.

The separator can be a porous membrane in some embodiments and/or anionomer membrane in other various embodiments. In some embodiments, theseparator can be formed from an ionically conductive polymer. Regardlessof its type, the separator or membrane can be ionically conductivetoward various ions.

Polymer membranes can be anion- or cation-conducting electrolytes. Wheredescribed as an “ionomer,” the term refers to polymer membranecontaining both electrically neutral repeating units and ionizedrepeating units, where the ionized repeating units are pendant andcovalently bonded to the polymer backbone. In general, the fraction ofionized units can range from about 1 mole percent to about 90 molepercent. For example, in some embodiments, the content of ionized unitsis less than about 15 mole percent; and in other embodiments, the ioniccontent is higher, such as greater than about 80 mole percent. In stillother embodiments, the ionic content is defined by an intermediaterange, for example, in a range of about 15 to about 80 mole percent.Ionized repeating units in an ionomer can include anionic functionalgroups such as sulfonate, carboxylate, and the like. These functionalgroups can be charge balanced by, mono-, di-, or higher-valent cations,such as alkali or alkaline earth metals. Ionomers can also includepolymer compositions containing attached or embedded quaternaryammonium, sulfonium, phosphazenium, and guanidinium residues or salts.Suitable examples will be familiar to one having ordinary skill in theart.

In some embodiments, polymers useful as a separator can include highlyfluorinated or perfluorinated polymer backbones. Certain polymers usefulin the present disclosure can include copolymers of tetrafluoroethyleneand one or more fluorinated, acid-functional co-monomers, which arecommercially available as NAFION™ perfluorinated polymer electrolytesfrom DuPont. Other useful perfluorinated polymers can include copolymersof tetrafluoroethylene and FSO₂—CF₂CF₂CF₂CF₂—O—CF═CF₂, FLEMION™ andSELEMION™.

Additionally, substantially non-fluorinated membranes that are modifiedwith sulfonic acid groups (or cation exchanged sulfonate groups) canalso be used. Such membranes can include those with substantiallyaromatic backbones such as, for example, polystyrene, polyphenylene,biphenyl sulfone (BPSH), or thermoplastics such as polyetherketones andpolyethersulfones.

Battery-separator style porous membranes, can also be used as theseparator. Because they contain no inherent ionic conductioncapabilities, such membranes are typically impregnated with additives inorder to function. These membranes typically contain a mixture of apolymer and inorganic filler, and open porosity. Suitable polymers caninclude, for example, high density polyethylene, polypropylene,polyvinylidene difluoride (PVDF), or polytetrafluoroethylene (PTFE).Suitable inorganic fillers can include silicon carbide matrix material,titanium dioxide, silicon dioxide, zinc phosphide, and ceria.

Separators can also be formed from polyesters, polyetherketones,poly(vinyl chloride), vinyl polymers, and substituted vinyl polymers.These can be used alone or in combination with any previously describedpolymer.

Porous separators are non-conductive membranes which allow chargetransfer between two electrodes via open channels filled withelectrolyte. The permeability increases the probability of activematerials passing through the separator from one electrode to anotherand causing cross-contamination and/or reduction in cell energyefficiency. The degree of this cross-contamination can depend on, amongother features, the size (the effective diameter and channel length),and character (hydrophobicity/hydrophilicity) of the pores, the natureof the electrolyte, and the degree of wetting between the pores and theelectrolyte.

The pore size distribution of a porous separator is generally sufficientto substantially prevent the crossover of active materials between thetwo electrolyte solutions. Suitable porous membranes can have an averagepore size distribution of between about 0.001 nm and 20 micrometers,more typically between about 0.001 nm to 100 nm. The size distributionof the pores in the porous membrane can be substantial. In other words,a porous membrane can contain a first plurality of pores with a verysmall diameter (approximately less than 1 nm) and a second plurality ofpores with a very large diameter (approximately greater than 10micrometers). The larger pore sizes can lead to a higher amount ofactive material crossover. The ability for a porous membrane tosubstantially prevent the crossover of active materials can depend onthe relative difference in size between the average pore size and theactive material. For example, when the active material is a metal centerin a coordination complex, the average diameter of the coordinationcomplex can be about 50% greater than the average pore size of theporous membrane. On the other hand, if a porous membrane hassubstantially uniform pore sizes, the average diameter of thecoordination complex can be about 20% larger than the average pore sizeof the porous membrane. Likewise, the average diameter of a coordinationcomplex is increased when it is further coordinated with at least onewater molecule. The diameter of a coordination complex of at least onewater molecule is generally considered to be the hydrodynamic diameter.In such embodiments, the hydrodynamic diameter is generally at leastabout 35% greater than the average pore size. When the average pore sizeis substantially uniform, the hydrodynamic radius can be about 10%greater than the average pore size.

In some embodiments, the separator can also include reinforcementmaterials for greater stability. Suitable reinforcement materials caninclude nylon, cotton, polyesters, crystalline silica, crystallinetitania, amorphous silica, amorphous titania, rubber, asbestos, wood orany combination thereof.

Separators within the flow batteries of the present disclosure can havea membrane thickness of less than about 500 micrometers, or less thanabout 300 micrometers, or less than about 250 micrometers, or less thanabout 200 micrometers, or less than about 100 micrometers, or less thanabout 75 micrometers, or less than about 50 micrometers, or less thanabout 30 micrometers, or less than about 25 micrometers, or less thanabout 20 micrometers, or less than about 15 micrometers, or less thanabout 10 micrometers. Suitable separators can include those in which theflow battery is capable of operating with a current efficiency ofgreater than about 85% with a current density of 100 mA/cm² when theseparator has a thickness of 100 micrometers. In further embodiments,the flow battery is capable of operating at a current efficiency ofgreater than 99.5% when the separator has a thickness of less than about50 micrometers, a current efficiency of greater than 99% when theseparator has a thickness of less than about 25 micrometers, and acurrent efficiency of greater than 98% when the separator has athickness of less than about 10 micrometers. Accordingly, suitableseparators include those in which the flow battery is capable ofoperating at a voltage efficiency of greater than 60% with a currentdensity of 100 mA/cm². In further embodiments, suitable separators caninclude those in which the flow battery is capable of operating at avoltage efficiency of greater than 70%, greater than 80% or even greaterthan 90%.

The diffusion rate of the first and second active materials through theseparator can be less than about 1×10⁻⁵ mol cm⁻² day⁻¹, or less thanabout 1×10⁻⁶ mol cm⁻² day⁻¹, or less than about 1×10⁻⁷ mol cm⁻² day⁻¹,or less than about 1×10⁻⁹ mol cm⁻² day⁻¹, or less than about 1×10⁻¹¹ molcm⁻² day⁻¹, or less than about 1×10⁻¹³ mol cm⁻² day⁻¹, or less thanabout 1×10⁻¹⁵ mol cm⁻² day⁻¹.

The flow batteries can also include an external electrical circuit inelectrical communication with the first and second electrodes. Theelectrical circuit can charge and discharge the flow battery duringoperation. Further exemplary embodiments of a flow battery provide that(a) the first active material has an associated net positive or negativecharge and is capable of providing an oxidized or reduced form over anelectric potential in a range of the negative operating potential of thesystem, such that the resulting oxidized or reduced form of the firstactive material has the same charge sign (positive or negative) as thefirst active material and the ionomer membrane also has a net ioniccharge of the same sign; and (b) the second active material has anassociated net positive or negative charge and is capable of providingan oxidized or reduced form over an electric potential in a range of thepositive operating potential of the system, such that the resultingoxidized or reduced form of the second active material has the samecharge sign (positive or negative sign) as the second active materialand the ionomer membrane also has a net ionic charge of the same sign;or both (a) and (b). The matching charges of the first and/or secondactive materials and the ionomer membrane can provide a highselectivity. More specifically, charge matching can provide less thanabout 3%, less than about 2%, less than about 1%, less than about 0.5%,less than about 0.2%, or less than about 0.1% of the molar flux of ionspassing through the ionomer membrane as being attributable to the firstor second active material. The term “molar flux of ions” refers to theamount of ions passing through the ionomer membrane, balancing thecharge associated with the flow of external electricity/electrons. Thatis, the flow battery is capable of operating with the substantialexclusion of the active materials by the ionomer membrane, and suchexclusion can be promoted through charge matching.

Flow batteries of the present disclosure can have one or more of thefollowing operating characteristics: (a) where, during the operation ofthe flow battery, the first or second active materials comprise lessthan about 3% of the molar flux of ions passing through the ionomermembrane; (b) where the round trip current efficiency is greater thanabout 70%, greater than about 80%, or greater than about 90%; (c) wherethe round trip current efficiency is greater than about 90%; (d) wherethe sign of the net ionic charge of the first, second, or both activematerials is the same in both oxidized and reduced forms of the activematerials and matches that of the ionomer membrane; (e) where theionomer membrane has a thickness of less than about 100 μm, less thanabout 75 μm, less than about 50 μm, or less than about 250 μm; (f) wherethe flow battery is capable of operating at a current density of greaterthan about 100 mA/cm² with a round trip voltage efficiency of greaterthan about 60%; and (g) where the energy density of the electrolytesolutions is greater than about 10 Wh/L, greater than about 20 Wh/L, orgreater than about 30 Wh/L.

In some cases, such as the embodiments discussed herein, a user maydesire to provide higher charge or discharge voltages than are availablefrom a single electrochemical cell. In such cases, severalelectrochemical cells can be connected in series such that the voltageof each cell is additive. This forms a bipolar stack, also referred toas an electrochemical cell stack herein. As discussed herein, a bipolarplate can be employed to connect adjacent electrochemical cells in abipolar stack, which allows for electron transport to take place butprevents fluid or gas transport between adjacent cells. The positiveelectrode compartments and negative electrode compartments of individualelectrochemical cells can be fluidically connected via inlet and outletmanifolds, as discussed herein. In this way, individual electrochemicalcells can be stacked in series to yield a voltage appropriate for DCapplications or conversion to AC applications.

In additional embodiments, the cells, bipolar stacks, or batteries canbe incorporated into larger energy storage systems, suitably includingpiping and controls useful for operation of these large units. Pipingand pumps provide fluidic conductivity for moving electrolyte solutionsinto and out of the respective chambers and storage tanks for holdingcharged and discharged electrolytes. The cells, cell stacks, andbatteries of this disclosure can also include an operation managementsystem. The operation management system can be any suitable controllerdevice, such as a computer or microprocessor, and can contain logiccircuitry that sets operation of any of the various valves, pumps,circulation loops, and the like.

In more specific embodiments, a flow battery system can include a flowbattery (including a cell or cell stack); storage tanks and piping forcontaining and transporting the electrolyte solutions control hardwareand software (which may include safety systems); and a powerconditioning unit. The cell stack accomplishes the conversion ofcharging and discharging cycles and determines the peak power. Thestorage tanks contain the positive and negative active materials, andthe tank volumes determine the quantity of energy stored in the system.The control software, hardware, and optional safety systems suitablyinclude sensors, mitigation equipment and other electronic/hardwarecontrols and safeguards to ensure safe, autonomous, and efficientoperation of the flow battery system. A power conditioning unit can beused at the front end of the energy storage system to convert incomingand outgoing power to a voltage and current that is optimal for theenemy storage system or the application. For the example of an energystorage system connected to an electrical grid, in a charging cycle thepower conditioning unit can convert, incoming AC electricity into DCelectricity at an appropriate voltage and current for the cell stack, ina discharging cycle, the stack produces DC electrical power and thepower conditioning unit converts it to AC electrical power at theappropriate voltage and frequency for grid applications.

Where not otherwise defined hereinabove or understood by one havingordinary skill in the art, the definitions in the following paragraphswill be applicable to the present disclosure.

As used herein, the term “energy density” refers to the amount of energythat can be stored, per unit volume, in the active materials. Energydensity refers to the theoretical energy density of energy storage andcan be calculated by Formula 1:Energy density=(26.8 A-h/mol)×OCV×[e ⁻]  (1)where OCV is the open circuit potential at 50% state of charge, (26.8A-h/mol) is Faraday's constant, and [e⁻] is the concentration ofelectrons stored in the active material at 99% state of charge. In thecase that the active materials largely are an atomic or molecularspecies for both the positive and negative electrolyte, [e⁻] can becalculated by Formula 2 as:[e ⁻]=[active materials]×N/2  (2)where [active materials] is the molar concentration of the activematerial in either the negative or positive electrolyte, whichever islower, and N is the number of electrons transferred per molecule ofactive material. The related term “charge density” refers to the totalamount of charge that each electrolyte contains. For a givenelectrolyte, the charge density can be calculated by Formula 3:Charge density=(26.8 A-h/mol)×[active material]×N  (3)where [active material] and n are as defined above.

As used herein, the term “current density” refers to the total currentpassed in an electrochemical cell divided by the geometric area of theelectrodes of the cell and is commonly reported in units of mA/cm².

As used herein, the term “current efficiency” (I_(eff)) is the ratio ofthe total charge produced upon discharge of a cell to the total chargepassed during charging. The current efficiency can be a function of thestate of charge of the flow battery. In some non-limiting embodiments,the current efficiency can be evaluated over a state of charge range ofabout 35% to about 60%.

As used herein, the term “voltage efficiency” is the ratio of theobserved electrode potential, at a given current density, to the halfcell potential for that electrode (×100%). Voltage efficiencies can bedescribed for a battery charging step, a discharging step, or a “roundtrip voltage efficiency.” The round trip voltage efficiency (V_(eff,RT))at a given current density can be calculated from the cell voltage atdischarge (V_(discharge)) and the voltage at charge (V_(charge)) usingFormula 4:V _(eff,RT) =V _(discharge) /V _(charge)×100%  (4)

As used herein, the terms “negative electrode” and “positive electrode”are electrodes defined with respect to one another, such that thenegative electrode operates or is designed or intended to operate at apotential more negative than the positive electrode (and vice versa),independent of the actual potentials at which they operate, in bothcharging and discharging cycles. The negative electrode may or may notactually operate or be designed or intended to operate at a negativepotential relative to a reversible hydrogen electrode. The negativeelectrode is associated with a first electrolyte solution and thepositive electrode is associated with a second electrolyte solution, asdescribed herein. The electrolyte solutions associated with the negativeand positive electrodes may be described as negolytes and posolytes,respectively.

Having now described flow batteries of the present disclosure in somedetail, illustrative methods for operating the flow batteries to provideimproved efficiency values will now be described.

In various embodiments, methods for regulating an electrolyte solutioncirculation rate in a flow battery can include: providing a flow batterycontaining: an electrochemical cell stack containing a plurality ofelectrochemical cells in electrical communication with one another, aninlet manifold containing an inflow channel fluidically connected to aninflow side of each of the electrochemical cells, and an outlet manifoldcontaining an outflow channel fluidically connected to an outflow sideof each of the electrochemical cells; placing an insert in the outflowchannel, and circulating an electrolyte solution through theelectrochemical cell stack via the inlet manifold and the outletmanifold. The insert has a variable width along a length of the outflowchannel. Particular configurations and dispositions of the insert arediscussed in more detail hereinabove.

In more particular embodiments, circulating the electrolyte solutionthrough the electrochemical cell stack can include: supplying theelectrolyte solution to the inflow channel longitudinally with respectto the plurality of electrochemical cells, circulating the electrolytesolution laterally through the plurality of electrochemical cells, andwithdrawing the electrolyte solution from the outflow channellongitudinally with respect to the plurality of electrochemical cells.Still more particularly, the electrolyte solution can be suppliedlongitudinally and withdrawn longitudinally via the inlet manifold andthe outlet manifold, respectively, from a single face of theelectrochemical cell stack.

When the insert is properly configured, the insert can levelize acirculation rate of the electrolyte solution through each of theelectrochemical cells. As used herein, the term “levelize” refers to thecondition of being made more uniform, particularly the condition ofmaking electrolyte circulation rates more uniform. In particularembodiments, a levelized circulation rate is such that circulation ratesthrough each of the electrochemical cells varies by less than about 5%,or less than about 2%, or less than about 1%, or less than 0.5%, or lessthan 0.25%, or less than 0.1%.

FIG. 10 shows a graph illustrating modeling of circulation rates throughthe individual electrochemical cells of an electrochemical cell stackwhen an insert is present versus when it is not. As shown in FIG. 10,the circulation rate variance is up to about ±1% when an insert is notpresent. In contrast, when a linearly tapered insert is present in theoutflow channel, the electrolyte solution circulation rates were muchmore tightly clustered within a circulation rate variance of about±0.1%. As shown, the circulation rate variance can become greater atcell positions located deeper in the electrochemical cell stack. FIG. 11shows a graph illustrating experimental discharge voltages forindividual electrochemical cells within an electrochemical cell stack asa function of position, in which an insert is not present. The variancein discharge voltages demonstrates the issues associated with varyingcirculation rates within a flow battery system. In particular,individual electrochemical cells having a lower circulation rate thereinoperate with a lower discharge voltage, since less active material ispresent over a given time interval.

As indicated above, methods of the present disclosure can also furtherinclude removing the insert from the outflow channel. Removal of theinsert can be conducted, for example, when the insert has failed or thecirculation conditions provided by the insert needed to be adjusted.Accordingly, in further embodiments, the insert disposed in the outflowchannel is a first insert and induces a first flow condition of theelectrolyte solution in the outflow channel, and the methods of thepresent disclosure can include removing the first insert from theoutflow channel, and inducing a second flow condition of the electrolytesolution in the outflow channel by placing a second insert in theoutflow channel, where the second insert is configured differently thanthe first insert.

In still other more specific embodiments, the insert can be affixed toflange that is operably connected to an outlet of the outflow channel,in which case placing the insert in the outflow channel includesconnecting the flange to the outlet of the outflow channel. Additionaldetails in this regard are provided hereinabove.

Although the disclosure has been described with reference to thedisclosed embodiments, those skilled in the art will readily appreciatethat these are only illustrative of the disclosure. It should beunderstood that various modifications can be made without departing fromthe spirit of the disclosure. The disclosure can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the disclosure. Additionally,while various embodiments of the disclosure have been described, it isto be understood that aspects of the disclosure may include only some ofthe described embodiments. Accordingly, the disclosure is not to be seenas limited by the foregoing description.

What is claimed is the following:
 1. A flow battery comprising: anelectrochemical cell stack comprising a plurality of electrochemicalcells in electrical communication with one another, each electrochemicalcell is configured to circulate electrolyte solution laterallytherethrough; an inlet manifold comprising an inflow channel having alength that is substantially constant in width and cross-section alongthe length and fluidically connected to an inflow side of each of theplurality of electrochemical cells and positioned longitudinally withrespect to the plurality of electrochemical cells, the inflow channelbeing configured to supply electrolyte solution to the plurality ofelectrochemical cells; an outlet manifold having a length, andcomprising an outflow channel fluidically connected to an outflow sideof each of the plurality of electrochemical cells and positionedlongitudinally with respect to the plurality of electrochemical cells,the outflow channel being configured to withdraw electrolyte solutionfrom the plurality of electrochemical cells through the outlet manifoldand out an outlet at one end of the outflow channel; and an insertdisposed in the outflow channel, the insert having a variable widthalong a length of the outflow channel, the insert having larger widthsat locations in the outflow channel that are more removed from theoutlet and smaller widths at locations nearer the outlet at the one endof the outflow channel.
 2. The flow battery of claim 1, wherein theinlet manifold and the outlet manifold are configured, respectively, tosupply and withdraw electrolyte solution longitudinally from a singleface of the electrochemical cell stack.
 3. The flow battery of claim 1,wherein the insert is tapered along the length of the outflow channel,and a width of the insert decreases in a direction configured forcirculation of the electrolyte solution in the outflow channel.
 4. Theflow battery of claim 1, wherein the insert is removably disposed withinthe outflow channel.
 5. The flow battery of claim 1, wherein the insertis affixed within the outflow channel.
 6. The flow battery of claim 1,further comprising a flange operably connected to the outlet of theoutflow channel, wherein the insert is affixed to the flange.
 7. Theflow battery of claim 1, wherein a first portion of the insert iscurved, and a second portion of the insert is linearly tapered.
 8. Theflow battery of claim 1, wherein the insert further comprises aturbulence inducer, a plurality of projecting hairs, a flow channel, abaffle, a fin, or any combination thereof.